The present invention relates to an improved startup device for the rotor of a permanent-magnet synchronous motor.
The device is particularly adapted for liquid conveyance pumps, such as those used in electric household appliances or the like.
In the field of electric household appliances or the like, pumps are conventionally used that are equipped with a permanent-magnet synchronous motor and must convey the washing fluids (lye) of said appliances.
The synchronous motor is composed of an electromagnet which is arranged outside the pump body and transmits an alternating electromagnetic field to the permanent magnet accommodated inside said pump body.
The variable electromagnetic flux, as known, imparts to the rotor, constituted by the permanent magnet, a torque that causes its rotation and accordingly, by virtue of motion transmission means, the rotation of the elements connected thereto.
In conventional pump models, especially in the simplest and cheapest ones, the rotor is rigidly coupled to the pump shaft by molding in place plastic material; the pump furthermore comprises disengagement devices by virtue whereof said shaft rotates without a load (freely) at least through a certain rotation angle, after which it starts to transmit motion to the impeller and therefore to the applied load.
Conventional systems for deferred actuation (or disengagement), despite performing their tasks, are not free from drawbacks.
Among these drawbacks, the most important are the mechanical wear and the seizure of said disengagement systems and therefore their deterioration, which is caused mainly by the abrasive action of the lye and of the aggressive substances contained therein.
The possibility of limiting this deterioration has been the subject of sometimes in-depth studies and researches aimed at solving the problem; the solutions that have been found are mainly based on protecting the disengagement system with systems such as:
dynamic sealing elements (for example lip seal rings or O-rings); however, since these elements are in turn subject to wear, they merely delay the contact between the lye and the disengagement elements and therefore do not solve the problem radically; PA0 labyrinth-type geometries of various kinds and shapes, used particularly in pumps of the above mentioned type, executed with the so-called "submersed-rotor" technology, which despite limiting the contact between the disengagement elements and the aggressive particles that have an excessive size do not protect said elements from hard water (scale-producing water) or from the microscopic impurities that are dissolved in the lye and cause, by accumulation, the seizure and wear of the moving parts. PA0 an increase in startup noise, caused by the impact between the mechanical parts that transmit motion to the load until the synchronous speed of the rotor has been reached; this increase is generated by the deterioration of the shock-absorbing systems generally provided to cushion the impact, at rotor startup, on the devices that transmit motion to the impeller; PA0 an increase in the noise level in operation in air and water, generated by the deterioration of the mechanical friction means (such as O-rings) provided in order to prevent early occurrence of the pole swapping of the rotating permanent magnet with respect to the movement of the impeller, which causes continuous knocking between said impeller and the rotor or, more precisely, the motion transmission means.
In any case, direct or indirect contact with conventional disengagement systems causes, in a short time, malfunctions of the pump, such as for example:
Further drawbacks to be mentioned are the early wear of the bushes provided for the movement of the shaft and therefore malfunctions and unwanted vibrations caused by the gradual increase in the imbalance of the rotor unit once said disengagement systems deteriorate, as well as the impossibility to maintain a constant production quality in the manufacture of the pump, mainly due to the difficulty of maintaining constant assembly tolerances.
A final drawback is caused by the fact that when these pumps are powered at lower-than-nominal voltages (as occurs in particular geographical regions of all countries), the perfect operation and the integrity of the disengagement system are essential; if they are not present, the rotor has difficulty in starting and knocking between the motion transmission elements again produces noise as well as pickup delay, with consequent loss of performance of the device in which the pump is included.